The present invention relates to a multi-wavelength semiconductor image sensor having sensitivity to infrared with plural wavelengths and also to a method of manufacturing the image sensor.
In multi-wavelength semiconductor image sensors, many photodetector elements made of, for example, photodiodes are arranged in the form of a matrix on a semiconductor substrate. As each photodetector element senses plural wavelengths, it has a structure corresponding to each wavelength.
As conventionally typical infrared image sensors using Hg1-xCdxTe and having sensitivity to plural wavelengths, there is a multi-wavelength semiconductor image sensor having a mesa structure which is disclosed in U.S. Pat. No. 5,113,076 (Santa Barbara Research Center, USA, 12.05.92). In this image sensor, if light is received from an n-type Hg0.7Cd0.3Te layer, a middle wavelength infrared is detected by a carrier produced in the vicinity of the reverse-biased junction between the n-type Hg0.7Cd0.3Te layer and a p-type Hg0.6Cd0.4Te layer whereas a long wavelength infrared is detected by a carrier produced in the vicinity of the reverse-biased junction between the p-type Hg0.6Cd0.4Te layer and an n-type Hg0.8Cd0.2Te layer.
Because this image sensor has a mesa structure, the fill factor of the indium doped n-type Hg0.8Cd0.2Te layer which is the second photo-absorbing layer is smaller than that of the indium doped n-type Hg0.7Cd0.3Te layer which is the first photo-absorbing layer. Therefore, only insufficient sensitivity to the wavelength corresponding to the n-type Hg0.8Cd0.2Te layer which is the second photo-absorbing layer is obtained.
Also, because a circuit which switches bias voltage to apply the voltage to a diode is required to select a wavelength band, there is the problem that a charge transfer device cannot be used by connecting it to the output.
In the image sensor disclosed in U.S. Pat. No. 5,149,956 (Santa Barbara Research Center, USA, 22.09.92), two photodiodes form one pixel which is provided with two interconnecting bumps. Because of this, it is difficult to reduce the area per one pixel and hence the pixel density cannot be improved.
While, in a multi-wavelength semiconductor detector disclosed in EP-A-0475525 (Philips Electronics and Associated Industries Limited, GB, 18.03.92), it is necessary that semiconductor layers which differ in the forbidden gap are thinned to tens of micron meters by a semiconductor layer polishing process and an etching process and then bonded with each other to form a laminated layer using an adhesive, bringing about remarkable production difficulty.
Also, two connecting electrodes per one pixel are required to draw a charge from a photo-absorbing layer, rendering high density integration difficult.
Moreover, because an isolation groove is formed in a photo-absorbing layer, the fill factor of the photo-absorbing layer which receives a long wavelength infrared is reduced with respect to a photo-absorbing layer which receives a middle wavelength infrared and hence only insufficient sensitivity can be obtained.
As stated above, as to conventional two-dimensional infrared image sensors using a compound semiconductor and having sensitivity to plural wavelengths, those having a structure suitable to high density integration have not been realized. Also its production process is complicated so that the objective sensor can be completed with difficulty.
It is an object of the present invention to provide a multi-wavelength semiconductor image sensor which enables high-density integration and can be made in a simplified process. It is another object of the present invention to provide a method of manufacturing the image sensor.
According to a first aspect of the present invention, there is provided a multi-wavelength semiconductor image sensor comprising a laminated structure having plural first conductive type semiconductor photo-absorbing layers which differ from each other in a forbidden gap and are formed on a substrate and a semiconductor isolation layer interposed between the first conductive type semiconductor photo-absorbing layers; a second conductive type region which is formed continuously in each of the first conductive type semiconductor photo-absorbing layers of the laminated structure and forms a pn-junction with the first conductive type semiconductor photo-absorbing layer; an output electrode connected to each of the second conductive type regions; and plural ground electrodes which are connected respectively to each of the first conductive type semiconductor photo-absorbing layers.
According to another aspect of the present invention, there is provided a multi-wavelength semiconductor image sensor comprising a first conductive type-first semiconductor photo-absorbing layer with a first forbidden gap which is formed on a substrate; a semiconductor isolation layer grown on the first semiconductor photo-absorbing layer; a first conductive type-second semiconductor photo-absorbing layer with a second forbidden gap differing from the first forbidden gap, the second semiconductor photo-absorbing layer being grown on the semiconductor isolation layer; plural second conductive type regions which are formed continuously in the first and second semiconductor photo-absorbing layers and forms a pn-junction with each of the first conductive type-first and second semiconductor photo-absorbing layers; an output electrode connected to each of the plural second conductive type regions; and a ground electrode connected to each of the first conductive type-first and second semiconductor photo-absorbing layers.
In the multi-wavelength semiconductor image sensor of the present invention, the forbidden gap of the semiconductor isolation layer is designed to be larger than any one of the forbidden gaps of the plural first conductive type semiconductor photo-absorbing layers with different forbidden gaps. Also, each surface of the second conductive type regions has a concave form. Impurities can be diffused to the deep position accordingly.
In the multi-wavelength semiconductor image sensor of the present invention, preferably a compound semiconductor including mercury is used for each of the first and second semiconductor photo-absorbing layers.
According to a further aspect of the present invention, there is provided a method of manufacturing a multi-wavelength semiconductor image sensor comprising a step of depositing a first conductive type-first semiconductor photo-absorbing layer with a first forbidden gap on a substrate; a step of depositing a semiconductor isolation layer, which is electrically isolated from the first semiconductor photo-absorbing layer, on the first semiconductor photo-absorbing layer; a step of etching the surface of the semiconductor isolation layer to form an opening portion; a step of introducing an impurity into the opening to form a second conductive type region; a step of depositing the next first conductive type semiconductor photo-absorbing layer with a forbidden gap differing from the first forbidden gap on the semiconductor isolation layer including the second conductive type region, a step of repeating the step of depositing the isolation layer, the step of forming the opening in the isolation layer and the step of forming the second conductive type region until the number of the laminated layers reaches a prescribed value; a step of forming an output electrode on the upper surface of the second conductive type region formed on the most upper semiconductor photo-absorbing layer; and a step of forming a ground electrode in each of the laminated first conductive type region as each semiconductor photo-absorbing layer.
According to a still further aspect of the present invention, there is provided a method of manufacturing a multi-wavelength semiconductor image sensor comprising a step of depositing a first conductive type semiconductor photo-absorbing layer with a first forbidden gap on a substrate; a step of depositing a semiconductor isolation layer, which is electrically isolated from the semiconductor photo-absorbing layer, on the semiconductor photo-absorbing layer; a step of etching the surface of the semiconductor isolation layer to form an opening portion; a step of depositing the next first conductive type semiconductor photo-absorbing layer with a forbidden gap differing from the first forbidden gap on the semiconductor isolation layer, a step of repeating the step of depositing the isolation layer, the step of forming the opening in the isolation layer and the step of depositing the next first conductive type semiconductor photo-absorbing layer with a different forbidden gap until the number of the laminated semiconductor photo-absorbing layers reaches a prescribed value; a step of introducing an impurity into the opening to form a second conductive type region penetrating each of the semiconductor photo-absorbing layers; a step of forming an output electrode on the upper surface of the second conductive type region; and a step of forming a ground electrode in each of the first conductive type semiconductor photo-absorbing layer.
According to a still further aspect of the present invention, there is provided a multi-wavelength semiconductor image sensor comprising photodiodes disposed at each position of pixels and sensing each of different wavelength regions; a first switching element to which each anode of the photodiodes disposed at each position of the pixels is connected in common; and a second switching element to which each cathode of photodiodes corresponding to the positions of plural pixels and sensing the same wavelength region every different wavelength region is connected in common. According to this multi-wavelength semiconductor image sensor, the pixels and wavelength regions can be optionally selected by switching these switching elements.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.